Various modern processes require the use of one-way valves. For example, in the processing and refining of various raw materials, the desired compounds or materials may be extracted, or undesired contaminants may be removed. One method of processing is by heating the material, causing a release of contaminants. Similarly, manufacturing processes such as sintering, may require the use of one-way valves to allow gas to escape during the heating process, which may also remove contaminants from the holding vessel and the starting materials.
For example, in manufacturing the cathode material of power batteries, the oxygen content in the atmosphere during the process of sintering lithium iron phosphate is preferably less than about 10 ppm. In conventional processes, the powder to be sintered into lithium iron phosphate is generally contained in an open-type container made of refractory material and the sintering device is preferably within an oxygen-free environment. This oxygen-free environment may cause various problems, such as instable quality and high operation cost. Alternatively, if the raw material is placed in an air-tight container for sintering, the presence of gases and other volatiles created during the pre-sintering process may contaminate the lithium iron phosphate. Information concerning the processing and properties of lithium iron phosphate may be found for example in Bewlay, S. L. et al., Conductivity improvements to spray produced LiFePO4 by addition of a carbon source, Materials Letters 58 (2004) 1788-1791, and Xie, Hui and Zhentao Zhou, Physical and electrochemical properties of mix-doped lithium iron phosphate as cathode material for lithium ion battery, Electrochimica Acta 51 (2006) 2063-2067.
At present, the research, development and production activities relating to lithium iron phosphate, as the cathode material for power batteries, are carried out on a large scale throughout the world. There is a consistent pursuit in the industry to reduce cost and ensure product quality. The sintering of lithium iron phosphate is one stage in the production process. The application of an air tight container in a common sintering furnace to sinter lithium iron phosphate is a recent development, whereby pure lithium iron phosphate emerges from the pre-sintering process without the emission of undesirable gases. The successful implementation of the technical process depends in part on the functional reliability of a one-way exhaust valve to maintain the purity of the lithium iron phosphate during the pre-sintering process.
Traditional one-way valves, or sealing valves typically rely on high geometric precision to ensure that the adjoining surfaces of the seal (or valve) closely coincide with each other, even under demanding conditions. For example, when a traditional one-way valve is configured to permit a flow of gas through the valve, the adjoining surfaces (or sealing faces) begin to wear once foreign material (e.g., dirt, pastes, viscous materials, etching/corrosive/acidic chemicals) passes through the valve along with the gas. In time, the continual wear will cause the seal to fail, rendering the one-way valve less effective. Further, traditional one-way valves use seals that may not survive harsh processing conditions, such as a high temperature environment.
Moreover, in applications with a strict sealing requirement, such as the processing of lithium iron phosphate (<10 ppm oxygen content), it may be very impractical and costly to use traditional one-way valves. First, due to the strict sealing requirement, the cost of the one-way valves may be high since the manufacturing of such a high precision and high performance valve is expensive. Second, as discussed above, foreign materials may still damage the valve and cause the valve to fail, which may require that a new valve be used for each pre-sintering run. Third, despite the strict sealing requirement, the one-way valve must still be responsive and flexible enough to allow proper gas flow, even when foreign materials become lodged or stuck within the sealing surfaces, obstructing the proper flow of gas, and also preventing a proper seal when the gas flow stops.